Resonant circuit and resonant dc/dc converter

ABSTRACT

The present disclosure relates to a resonant circuit ( 20 ). The resonant circuit comprises three resonant circuit input nodes ( 11, 12, 13 ) and three resonant circuit output nodes ( 21, 22, 23 ), a transformer device and resonant tank devices. The transformer device (TR) comprises three primary windings (LP 1,  LP 2,  LP 3 ) and three secondary windings (LS 1,  LS 2,  LS 3 ) magnetically connected to each other, where the three secondary windings (LS 1,  LS 2,  LS 3 ) are connected to the three resonant circuit output nodes ( 21, 22, 23 ). The first, second and third resonant tank devices (RT 1,  RT 2,  RT 3 ) are each connected between the respective three resonant circuit input nodes ( 11, 12, 13 ) and the respective primary windings (LP 1,  LP 2,  LP 3 ). The disclosure also relates to a resonant DC-DC converter comprising such a resonant circuit ( 20 ).

FIELD OF THE INVENTION

The present invention relates to a resonant circuit and a resonant DC/DCconverter.

BACKGROUND OF THE INVENTION

High efficiency galvanic isolation is needed in many power electronicapplications. Transformer based isolation may be needed due to safety,change of voltage levels, or functional issues. Some renewable energysources need galvanic isolation to work properly; among them aredifferent types of thin film solar panels.

A solar cell panel generates DC power. To supply the DC power to an ACload, either directly or via an AC power distribution network, a powerconverter system must be connected between the solar cell panel and theAC load for converting the DC power to AC power. Such power convertersystems normally comprise a DC/DC converter and a DC/AC converter, theDC/AC converter normally being referred to as an inverter. In addition,the system comprises a control system for controlling the converters,and other components such as filters, fuses, cooling systems etc.

The DC output power generated by a solar cell panel is changing with sunintensity and temperature. High conversion efficiency is very importantto maximize energy harvest from a solar plant in the context of makingthe investment profitable.

The series resonant LLC DC/DC converter has become a popular alternativeto PWM type converters in many applications. One advantage with theresonant converter is that it can be designed for high efficiency forall load and input/output voltage conditions since it can maintainzero-voltage switching for all operating conditions.

The practical limitation for a single resonant LLC converter is set bythe increasing size and cost of the resonant inductor as the outputpower increases. Therefore, high power DC/DC-converters are provided asmany smaller series resonant LLC converters in parallel, which increasethe cost considerably due to the number of components.

The object of the present invention is to provide a resonant circuitwith high efficiency and low cost, and which has high efficiency for awide input voltage range. Moreover, the object is to reduce the numberof components, and thereby reducing the complexity and costs involved.In addition, it is an object of the invention to reduce the ripplecurrents generated by the converter.

The object of the invention is also to provide a resonant DC/DCconverter with such a resonant circuit.

SUMMARY OF THE INVENTION

The present invention relates to a resonant circuit comprising threeresonant circuit input nodes and three resonant circuit output nodes; atransformer device comprising three primary windings and three secondarywindings magnetically connected to each other, where the three secondarywindings are connected to the three resonant circuit output nodes;first, second and third resonant tank devices each connected between therespective three resonant circuit input nodes and the respective primarywindings; first, second and third transformer switching devices forreconfiguring the three secondary windings between a delta-configurationand a star-configuration.

In one aspect each resonant tank device comprises a resonant inductorand a resonant capacitor.

In one aspect the three primary windings together with the threeresonant tank devices are configured in a delta-configuration.

In one aspect first, second and third transformer switching devices areprovided for reconfiguring the three primary windings between adelta-configuration and a star-configuration.

In one aspect the first transformer switching device comprises a commonterminal connected to second terminal of the second transformerswitching device, a first terminal connected to the first resonantoutput node and a second terminal connected to the common terminal ofthe third transformer switching device; the second transformer switchingdevice comprises a common terminal connected to second terminal of thethird transformer switching device, a first terminal connected to thesecond resonant circuit output node and a second terminal connected tothe common terminal of the first transformer switching device; the thirdtransformer switching device comprises the common terminal connected thesecond terminal of the first transformer switching device, a firstterminal connected to the third resonant circuit output node and asecond terminal connected to the common terminal of the secondtransformer switching device; where the first secondary winding isconnected between the first and second terminals of the firsttransformer switching device, the second secondary winding is connectedbetween the first and second terminals of the second transformerswitching device and the third secondary winding is connected betweenthe first and second terminals of the third transformer switchingdevice, where the transformer device is connected in adelta-configuration when the first terminals of the respectivetransformer switching devices are connected to their common terminal andwhere the transformer device is connected in a star-configuration whenthe second terminals of the respective transformer switching devices areconnected to their common terminals.

In one aspect the first resonant inductor, the first resonant capacitorand the first primary winding are connected in series between the firstresonant circuit input node and the third resonant circuit input node;the second resonant inductor, the second resonant capacitor and thesecond primary winding are connected in series between the secondresonant circuit input node and the first resonant circuit input node;and the third resonant inductor, the third resonant capacitor and thethird primary winding are connected in series between the third resonantcircuit input node and the second resonant circuit input node.

In one aspect a magnetic inductor is connected in parallel with each ofthe primary windings.

In one aspect the first resonant inductor, the first resonant capacitorand the first primary winding are connected in series between the firstresonant circuit input node and a primary common node; the secondresonant inductor, the second resonant capacitor and the second primarywinding are connected in series between the second resonant circuitinput node and a primary common node; and the third resonant inductor,the third resonant capacitor and the third primary winding are connectedin series between the third resonant circuit input node and a primarycommon node.

The invention also relates to a resonant DC-DC converter comprisingfirst and second input terminals and first and second output terminals;a switching device connected between the first and second inputterminals and three resonant circuit input nodes of a resonant circuit;a rectifier device connected between three resonant circuit output nodesand the first and second output terminals; where the resonant circuitcomprises a transformer device comprising three primary windings andthree secondary windings magnetically connected to each other, where thethree secondary windings are connected to the three resonant circuitoutput nodes; where the resonant circuit comprises first, second andthird resonant tank devices each connected between the respective threeresonant circuit input nodes and the respective primary windings; andwhere the resonant circuit comprises first, second and third transformerswitching devices for reconfiguring the three secondary windings betweena delta-configuration and a star-configuration.

In one aspect each resonant tank device comprises a resonant inductorand a resonant capacitor.

In one aspect the three primary windings are configured in adelta-configuration.

In one aspect first, second and third transformer switching devices forreconfiguring the three primary windings between a delta-configurationand a star-configuration.

In one aspect the switching device comprises six switching devices,where each switching device is connected between one of the first orsecond input terminals and one of the respective switch output nodes.

In one aspect the rectifier device is a diode rectifier or a synchronousrectifier.

DETAILED DESCRIPTION

In the following, embodiments of the invention will be described withreference to the enclosed drawings, where:

FIG. 1 illustrates a power converter system for converting DC power froma solar cell panel to AC power supplied to an AC power distributionnetwork or an AC load;

FIG. 2 is a schematic block diagram of the DC/DC converter of FIG. 1;

FIG. 3 is a first embodiment of a resonant DC/DC converter;

FIG. 4 is a second embodiment of a resonant DC/DC converter;

FIG. 5 is a third embodiment of a resonant DC/DC converter;

FIG. 6 is a fourth embodiment of a resonant DC/DC converter;

FIG. 7 is a fifth embodiment of a resonant DC/DC converter;

FIG. 8 is a sixth embodiment of a resonant DC/DC converter;

FIG. 9 is a seventh embodiment of a resonant DC/DC converter;

FIG. 10 is an eight embodiment of a resonant DC/DC converter;

FIG. 11 is a ninth embodiment of a resonant DC/DC converter;

FIG. 12 is an illustration of a prior art converter typically used insuch applications, comprising two series resonant LLC converters inparallel phase shifted 90 degrees;

FIG. 13 a shows the results of a simulation of the circuit in FIG. 12;and

FIG. 13 b shows the results of a simulation of the circuit in FIG. 3.

It is now referred to FIG. 1. A power converter system 1 is connectedbetween a DC power source 2 and an AC load or AC distribution network 3.The DC power source is here a solar cell panel or a module comprisingseveral solar cell panels, but can be any other type of suitable energysource.

The power converter system converts the input DC power to an AC outputpower. The power converter system comprises a DC/DC converter and aninverter (DC/AC converter), filters etc as mentioned in the descriptionabove. Moreover, the system comprises a control system for controllingthe DC/DC converter, the inverter and other components. The system inFIG. 1 is in general known for a skilled person.

The present invention relates to the DC/DC converter in FIG. 1. It isnow referred to FIG. 2. FIG. 2 illustrates a resonant DC/DC convertercomprising a switching device 10, a resonant circuit 20 and a rectifier30 connected to each other between first and second input terminals IT1,IT2 and first and second output terminals OT1, OT2.

The switching device 10 comprises six switches S1, S2, S3, S4, S5, S6,where each switch is connected between one of the first or second inputterminals IT1, IT2 and one of the respective input nodes of the resonantcircuit, hereinafter referred to as resonant circuit input nodes 11, 12,13.

The first switch S1 is connected between the first input terminal IT1and the first switch output terminal 11, a second switch S2 is connectedbetween the first switch output node 11 and the second input terminalIT2, a third switch S3 is connected between the first input terminal IT1and the second switch output terminal 12, a fourth switch S4 isconnected between the second switch output node 12 and the second inputterminal IT2, a fifth switch S5 is connected between the first inputterminal IT1 and the third switch output terminal 13 and a sixth switchS6 is connected between the third switch output node 13 and the secondinput terminal IT2. The switches S1, S2, S3, S4, S5, and S6 are MOSFETswitches. Alternatively, the switches may be switches with intrinsicdiodes or switches connected in parallel with anti-parallel diodes.

The switching device 10 further comprises switch capacitors CS1, CS2,CS3, CS4, CS5 and CS6, each connected in parallel with one of therespective switches 51, S2, S3, S4, S5, S6.

The switches S1, S2, S3, S4, S5, and S6 are controlled by the controlsystem illustrated in FIG. 1. It should be noted that the switches S1-S6can be controlled by frequency, PWM, or as a hybrid of both of these.

As mentioned above, the resonant circuit 20 comprises three resonantcircuit input nodes 11, 12, 13. The resonant circuit 20 also comprisesthree resonant circuit output nodes 21, 22, 23.

The rectifier device 30 is connected between the three resonant circuitoutput nodes 21, 22, 23 and the first and second output terminals OT1,OT2. The rectifier device 30 of FIG. 2 is a diode rectifier.

The rectifier device 30 of FIG. 2 comprises a first diode D1 with itsanode connected to the first rectifier input node 21 and its cathodeconnected to the first output terminal OT1, a second diode D2 with itsanode connected to the second output terminal OT2 and its cathodeconnected to the first rectifier input node 21, a third diode D3 withits anode connected to the second rectifier input node 22 and itscathode connected to the first output terminal OT1, a fourth diode D4with its anode connected to the second output terminal OT2 and itscathode connected to the second rectifier input node 22, a fifth diodeD5 with its anode connected to the third rectifier input node 23 and itscathode connected to the first output terminal OT1 and a sixth diode D6with its anode connected to the second output terminal OT2 and itscathode connected to the third rectifier input node 23.

Alternatively, the rectifier device 30 may be a synchronous rectifier.

In FIG. 2, it is shown that an output capacitor Cout is connectedbetween the first and second output terminals OT1, OT2.

It should be noted that the switching device 10 and the rectifier device30 are considered known for a skilled person. Also the control of theswitches in the switching device 10 is considered known for a skilledperson. The control of the switches is based on soft switching orso-called zero voltage switching (ZVS), where the voltage over theswitch is equal to or near zero V when the switch is turned on/off.

The resonant circuit 20 according to the invention will now be describedwith reference to FIG. 2. The resonant circuit comprises a transformerdevice TR comprising three primary windings LP1, LP2, LP3 and threesecondary windings LS1, LS2, LS3 magnetically connected to each other,where the three secondary windings LS1, LS2, LS3 are connected to thethree resonant circuit output nodes 21, 22, 23. Moreover, the resonantcircuit 20 comprises first, second and third resonant tank devices RT1,RT2, RT3 each connected between the respective three resonant circuitinput nodes 11, 12, 13 and the respective primary windings LP1, LP2,LP3.

The transformer device TR may be a three phase transformer. In analternative embodiment, also three single phased transformers may beused. The primary windings LP1, LP2, LP3 may be configured in adelta-configuration or in a star-configuration. More precisely, thethree primary windings LP1, LP2, LP3 together with the three resonanttank devices RT1, RT2, RT3 may be configured in a delta-configuration orin a star configuration, as will be described in detail below. Thesecondary windings LS1, LS2, LS3 may be configured in adelta-configuration or in a star-configuration.

As will be described below, the resonant circuit 20 comprises first,second and third transformer switching devices ST1, ST2, ST3 forreconfiguring the three secondary windings LS1, LS2, LS3 between adelta-configuration and a star-configuration.

Moreover, the resonant circuit 20 may comprise first, second and thirdtransformer switching devices ST1, ST2, ST3 for reconfiguring the threeprimary windings LP1, LP2, LP3 between a delta-configuration and astar-configuration.

The transformer switching devices ST1, ST2, ST3 may be controlled by thecontrol system.

Each resonant tank device RT1, RT2, RT3 comprises a resonant inductorLR1, LR2, LR3 and a resonant capacitor CR1, CR2, CR3. The resonant tankdevice, together with the primary windings LP1, LP2, LP3 providesresonance for the zero-voltage switching of the switching device 10. Theresonant tank devices may be partially or fully integrated in thetransformer device, utilizing the leakage inductance and other parasiticelements.

The voltage between the first and second input terminals IT1 and IT2 isreferred to as Uin. The voltage between the first and second outputterminals OT1 and OT2 is referred to as Uout.

It should be noted that the voltage and/or current in each of the threebranches of the resonant device 20 has the same magnitude, but aredisplaced in time by 120 electrical degrees. Consequently, the device 20together with devices 10 and 30 are a three phase resonant DC/DCconverter.

It should be noted that in the embodiments described below, the first,second and third transformer switching devices ST1, ST2, ST3 forreconfiguring the three secondary windings LS1, LS2, LS3 between adelta-configuration and a star-configuration are not shown in alldrawings. However, the secondary windings LS1, LS2, LS3 are either in adelta-configuration or in a star-configuration in all embodiments below.

First Embodiment

It is now referred to FIG. 3. Here, the first resonant tank devicecomprises the first resonant inductor LR1 and the first resonantcapacitor CR1 connected in series with the first primary winding LP1between the first resonant circuit input node 11 and the third resonantcircuit input node 13. The second resonant tank device comprises thesecond resonant inductor LR2 and the second resonant capacitor CR2connected in series with the second primary winding LP2 between thesecond resonant circuit input node 12 and the first resonant circuitinput node 11. The third resonant tank device comprises the thirdresonant inductor LR3 and the third resonant capacitor CR3 connected inseries with the third primary winding LP3 between the third resonantcircuit input node 13 and the second resonant circuit input node 12.

The secondary windings LS1, LS2, LS3 are connected between the resonantcircuit output nodes 21, 22, 23. The first secondary winding LS1 isconnected between the first and second rectifier input nodes 21, 22, thesecond secondary winding LS2 is connected between the second and thirdrectifier input nodes 22, 23 and the third secondary winding LS3 isconnected between the third and first rectifier input nodes 23, 21.

Consequently, the primary side of the transformer device TR is connectedin a delta-configuration and the secondary side of the transformerdevice TR is connected in a delta-configuration. i.e. the first, secondand third transformer switching devices ST1, ST2, ST3 are configured sothat the three secondary windings LS1, LS2, LS3 are in adelta-configuration. It should be noted that the term “primary side”here denotes the primary windings and the components of the resonanttank. Hence, the three primary windings LP1, LP2, LP3 together with thethree resonant tank devices RT1, RT2, RT3 are configured in adelta-configuration.

It should be noted that also here the resonant tank devices may bepartially or fully integrated in the transformer device, as describedabove.

Second Embodiment

It is now referred to FIG. 4.

Here, the resonant tank devices and the primary side of the transformerdevice are configured as in the first embodiment above.

The secondary windings are here also connected between the resonantcircuit output nodes 21, 22, 23. A secondary common node 24 is shown inthe FIG. 4. The first secondary winding LS1 is connected between thefirst resonant circuit output node 21 and the secondary common node 24.The second secondary winding LS2 is connected between the secondresonant circuit output node 22 and the secondary common node 24. Thethird secondary winding LS3 is connected between the third resonantcircuit output node 23 and the secondary common node 24.

Consequently, the primary side of the transformer device TR is connectedin a delta-configuration and the secondary side of the transformerdevice TR is connected in a star-configuration, i.e. the first, secondand third transformer switching devices ST1, ST2, ST3 are configured sothat the three secondary windings LS1, LS2, LS3 are in astar-configuration. It should be noted that the term “primary side” heredenotes the primary windings and the components of the resonant tank.Hence, the three primary windings LP1, LP2, LP3 together with the threeresonant tank devices RT1, RT2, RT3 are configured in adelta-configuration.

The secondary common node 24 may be considered as the common point ofthe star-configured transformer. Note that the term “secondary” is hereused to denote the location on the secondary side of the transformerdevice.

Also here it should be noted that the resonant tank devices may bepartially or fully integrated in the transformer device.

Third Embodiment

It is now referred to FIG. 5 a and FIG. 5 b.

Here, the resonant tank devices and the primary side of the transformerdevice is configured as in the first embodiment above.

The secondary windings are here also connected between the resonantcircuit output nodes 21, 22, 23. However, here the resonant circuit 20comprises first, second and third transformer switching devices ST1,ST2, ST3 for reconfiguring the three secondary windings LS1, LS2, LS3between a delta-configuration and a star-configuration. The terminologyof the first transformer switching device ST1 is shown in FIG. 5 b. Theswitching of the transformer switching devices ST1, ST2, ST3 may beperformed by means of relay devices, or any other switching devices,controlled by the control system.

The first transformer switching device ST1 comprises a common terminalTcom connected to a second terminal T2 of the second transformerswitching device ST2, a first terminal T1 connected to the firstresonant circuit output node 21 and a second terminal T2 connected tothe common terminal Tcom of the third transformer switching device ST3.

The second transformer switching device ST2 comprises a common terminalTcom connected to a second terminal T2 of the third transformerswitching device ST3, a first terminal T1 connected to the secondresonant circuit output node 22 and a second terminal T2 connected tothe common terminal Tcom of the first transformer switching device ST1.

The third transformer switching device ST3 comprises the common terminalTcom connected the second terminal T2 of the first transformer switchingdevice ST1, a first terminal T1 connected to the third resonant circuitoutput node 23 and a second terminal T2 connected to the common terminalTcom of the second transformer switching device ST2.

The first secondary winding LS1 is connected between the first andsecond terminals T1, T2 of the first transformer switching device ST1,the second secondary winding LS2 is connected between the first andsecond terminals T1, T2 of the second transformer switching device ST2and the third secondary winding LS3 is connected between the first andsecond terminals TI, T2 of the third transformer switching device ST3.

The transformer device TR is connected in a delta-delta configurationwhen the first terminals T1 of the respective transformer switchingdevices are connected to their common terminal Tcom. The transformerdevice TR is connected in a delta-star configuration when the secondterminals T2 of the respective transformer switching devices areconnected to their common terminals Tcom.

The transformer switching devices ST1, ST2, ST3 are be controlled by thecontrol system. For example, the input voltage Uin may be measured andgiven as an input to the control system. If the measured input voltageUin is below a certain threshold value, the transformer switchingdevices ST1, ST2, ST3 is switched so that the secondary side of thetransformer device is connected in a star configuration. If the measuredinput voltage Uin is above a certain threshold value, the transformerswitching devices ST1, ST2, ST3 is switched so that the secondary sideof the transformer device is connected in a delta-configuration.Consequently, the output voltage Uout decreases.

In an alternative embodiment, such transformer switching devices ST1,ST2, ST3 may be provided on the primary side in addition to thetransformer switching devices ST1, ST2, ST3 on the secondary side.Hence, the primary side of the transformer device may be reconfiguredbetween a delta-configuration and a star-configuration.

Fourth Embodiment

It is now referred to FIG. 6.

The resonant circuit 20 is here similar to the resonant circuit 20 ofthe second embodiment (FIG. 4).

Here, the rectifier device 30 is not a diode rectifier. Instead, therectifier device 30 is a synchronous rectifier with six switches S7, S8,S9, S10, S11, S12 instead of the six diodes. The rectifier devicecomprises six switch capacitors CS7, CS8, CS9, CS10, CS11 and CS12 eachconnected in parallel to one of the six switches.

Also these switches are MOSFET switches. Alternatively, the switches maybe switches with intrinsic diodes or switches connected in parallel withanti-parallel diodes.

It should be noted that this type of rectifier device 30 may be used forany of the above embodiments and any of the embodiments below. Moreover,it may be used for any configuration of the transformer device(delta-delta, delta-star, star-delta, star-star).

It should be also be noted that with this type of rectifier device, itwould be possible to achieve bidirectional power flow, i.e. power mayflow from input terminals to output terminals, but also from outputterminals to input terminals as defined in FIG. 6. This type ofconverter is normally referred to as a bidirectional dc-dc converter.

Fifth Embodiment

It is now referred to FIG. 7. The resonant circuit 20 is here similar tothe resonant circuit of FIG. 6 and FIG. 4. Here, the resonant circuit 20further comprises one magnetizing inductor Lm1, Lm2, Lm3 connected inparallel with each of the primary windings LP1, LP2, LP3. Hence, thethree primary windings LP1, LP2, LP3 together with the three resonanttank devices RT1, RT2, RT3 are configured in a star-configuration.

A first magnetizing inductor Lm1 is connected in parallel with the firstprimary winding LP1. A second magnetizing inductor Lm2 is connected inparallel with the second primary winding LP2. A third magnetizinginductor Lm3 is connected in parallel with the third primary windingLP3. The magnetizing inductor will influence on the resonance of theresonant circuit.

The magnetizing inductors may be magnetic coupled inductor (threephase), it may be three single inductors, or it may be fully integratedin the transformer device.

Sixth Embodiment

It is now referred to FIG. 8. Here, the first resonant tank devicecomprises the first resonant inductor LR1 and the first resonantcapacitor CR1 connected in series with the first primary winding LP1between the first resonant circuit input node 11 and a primary commonnode 25. The second resonant tank device comprises the second resonantinductor LR2 and the second resonant capacitor CR2 connected in serieswith the second primary winding LP2 between the second resonant circuitinput node 12 and the primary common node 25. The third resonant tankdevice comprises the third resonant inductor LR3 and the third resonantcapacitor CR3 connected in series with the third primary winding LP3between the third resonant circuit input node 13 and the primary commonnode 25.

Consequently, the primary side of the transformer device is hereconfigured as a star-configuration. More precisely, the three primarywindings LP1, LP2, LP3 together with the three resonant tank devicesRT1, RT2, RT3 are configured in a star-configuration.

The secondary side of the transformer device (i.e. the secondarywindings) are here connected as a star-configuration as in theembodiment shown and described in FIG. 4. Consequently, the first,second and third transformer switching devices ST1, ST2, ST3 areconfigured so that the three secondary windings LS1, LS2, LS3 are in astar-configuration.

Seventh Embodiment

It is now referred to FIG. 9.

The primary side of the transformer device is here connected as astar-configuration as described with reference to FIG. 8 above.

The secondary side of the transformer device is connected as adelta-configuration as described above with reference to FIG. 3 and FIG.7. Consequently, the first, second and third transformer switchingdevices ST1, ST2, ST3 are configured so that the three secondarywindings LS1, LS2, LS3 are in a delta-configuration.

Eight Embodiment

It is now referred to FIG. 10.

Here, the first resonant inductor LR1 and the first resonant capacitorCR1 are connected in series between the first resonant circuit inputnode 21 and the primary common node 25. The first primary winding LP1 isconnected in parallel with the first resonant capacitor CR1.

The second resonant inductor LR2 and the second resonant capacitor CR2are connected in series between the second resonant circuit input node22 and the primary common node 25. The second primary winding LP2 isconnected in parallel with the second resonant capacitor CR2.

The third resonant inductor LR3 and the third resonant capacitor CR3 areconnected in series between the third resonant circuit input node 23 andthe primary common node 25. The third primary winding LP3 is connectedin parallel with the third resonant capacitor CR3.

Hence, the three primary windings LP1, LP2, LP3 together with the threeresonant tank devices RT1, RT2, RT3 are configured in astar-configuration.

The primary side of the transformer device is here connected as astar-configuration. The secondary side of the transformer device is hereconnected as a star-configuration. Consequently, the first, second andthird transformer switching devices ST1, ST2, ST3 are configured so thatthe three secondary windings LSI, LS2, LS3 are in a star-configuration.

Ninth Embodiment

It is now referred to FIG. 11. The primary side of the transformerdevice is here connected as a star-configuration as described withreference to FIG. 10 above. The secondary side of the transformer deviceis connected as a delta-configuration as described with reference toFIGS. 3, 7 and 9 above.

Test Results

As mentioned in the introduction, the most common prior art circuit forsuch types of resonant DC/DC converters are two series resonant LLCconverters, as illustrated in FIG. 12 a.

A simulation of the prior art circuit in FIG. 12 has been performed byusing LTspice from Linear Technology (http://www.linear.com). In thesimulation, the input and output values for the simulation were Uout=350Vdc, Iout=17 Adc, Uin=350 Vdc.

The result of the simulation is shown in FIG. 13 a. Here it is shownthat the input ripple current IV2 here has a peak-to-peak value ofapproximately 10 A.

A corresponding simulation of the circuit shown in FIG. 3 was alsoperformed in the same way as above, with the same input and outputvalues.

The result of this simulation is shown in FIG. 13 b. From the results ofFIG. 13 b the input ripple current IV1 here has a peak-to-peak value ofapproximately 5 A.

Consequently, the input ripple current has been clearly reduced for thepresent invention compared with prior art.

Moreover, it can be seen that the converter of FIG. 3 has a lot fewercomponents than the converter of FIG. 12. According to the three phasedesign of the resonant tank device and transformer device, the currentof each branch is decreased, which reduces the losses.

It is also achieved a circuit with large flexibility with respect to thecontrollable ratio of the input voltage/output voltage. The low currentthrough the resonant tank makes it Well suited for a high impedanceresonant tank. In addition, reconfiguration of the transformer deviceand resonant tank device for example from a delta-delta-configuration toa delta-star-configuration for low input voltages improves thiscontrollable ratio further.

The low ripple current also eliminates the need for electrolyticcapacitors.

It should be noted that the high efficiency may be increased further atlow power by stop switching one of the three branches on the primaryside, for example by keeping the switches S5 and S6 constant off. Theconverter will then act as a quasi full bridge resonant converter.

1. A resonant circuit comprising: a first resonant circuit input node, asecond resonant circuit input node, and a third resonant circuit inputnode; a first resonant circuit output node, a second resonant circuitoutput node, and a third resonant circuit output node; a transformerdevice comprising: a first primary winding, a second primary winding,and a third primary winding; a first secondary winding, a secondsecondary winding, and a third secondary winding, wherein the first,second, and third primary windings are magnetically connected to thefirst, second, and third secondary windings each other, and wherein thefirst, second, and third secondary windings are connected to the first,second, and third resonant circuit output nodes; a first resonant tankdevice, a second resonant tank device, and a third resonant tank deviceeach connected between the respective resonant circuit input nodes andthe respective primary windings; and a first transformer switchingdevice, a second transformer switching device, and a third transformerswitching device arranged to reconfigure the first, second, and thirdsecondary windings between a delta-configuration and astar-configuration, wherein the first, second, and third primarywindings together with the first, second, and third resonant tankdevices are configured in the delta-configuration.
 2. The resonantcircuit according to claim 1, wherein each resonant tank devicecomprises a resonant inductor and a resonant capacitor.
 3. (canceled) 4.(canceled)
 5. (canceled)
 6. The resonant circuit according to claim 2,wherein: the resonant inductor of the first resonant tank device, theresonant capacitor of the first resonant tank device, and the firstprimary winding are connected in series between the first resonantcircuit input node and the third resonant circuit input node, theresonant inductor of the second resonant tank device, the resonantcapacitor of the second resonant tank device, and the second primarywinding are connected in series between the second resonant circuitinput node and the first resonant circuit input node, and the resonantinductor of the third resonant tank device, the resonant capacitor ofthe third resonant tank device, and the third primary winding areconnected in series between the third resonant circuit input node andthe second resonant circuit input node.
 7. The resonant circuitaccording to claim 2, wherein a magnetic inductor is connected inparallel with each of the primary windings.
 8. The resonant circuitaccording to claim 2, wherein: the resonant inductor of the firstresonant tank device, the resonant capacitor of the first resonant tankdevice, and the first primary winding are connected in series betweenthe first resonant circuit input node and a primary common node, theresonant inductor of the second resonant tank device, the resonantcapacitor of the second resonant tank device, and the second primarywinding are connected in series between the second resonant circuitinput node and the primary common node, and the resonant inductor of thethird resonant tank device, the resonant capacitor of the third resonanttank device, and the third primary winding are connected in seriesbetween the third resonant circuit input node and the primary commonnode.
 9. A resonant DC-DC converter comprising: a first input terminaland a second input terminal; a first output terminal and a second outputterminal; a switching device connected between the first and secondinput terminals and a first resonant circuit input node, a secondresonant circuit input node, and a third resonant circuit input node ofa resonant circuit; a rectifier device connected between a firstresonant circuit output node, a second resonant circuit output node, anda third resonant circuit output node and the first and second outputterminals, wherein the resonant circuit comprises a transformer devicecomprising: a first primary winding, a second primary winding, and athird primary winding; a first secondary winding, a second secondarywinding, and a third secondary winding, wherein the first, second, andthird primary windings are magnetically connected to the first, second,and third secondary windings, wherein the the first, second, and thirdsecondary windings are connected to the first, second, and thirdresonant circuit output nodes, wherein the resonant circuit comprises afirst resonant tank device, a second resonant tank device, and a thirdresonant tank device each connected between the respective first,second, and third resonant circuit input nodes and the respective first,second, and third primary windings, and wherein the resonant circuitcomprises a first transformer switching device, a second transformerswitching device, and a third transformer switching device arranged toreconfigure the first, second, and third secondary windings between adelta-configuration and a star-configuration and the first, second, andthird primary windings are configured in a delta-configuration.
 10. Theconverter according to claim 9, wherein each resonant tank devicecomprises a resonant inductor and a resonant capacitor.
 11. (canceled)12. (canceled)
 13. The converter according to claim 9, wherein theswitching device comprises six switching devices, wherein each switchingdevice is connected between one of the first or second input terminalsand one of the respective switch output nodes.
 14. The converteraccording to claim 9, wherein the rectifier device is a diode rectifieror a synchronous rectifier.
 15. The converter according to claim 10,wherein the switching device comprises six switching devices, whereineach switching device is connected between one of the first or secondinput terminals and one of the respective switch output nodes.
 16. Theconverter according to claim 10, wherein the rectifier device is a dioderectifier or a synchronous rectifier.
 17. The converter according toclaim 13, wherein the rectifier device is a diode rectifier or asynchronous rectifier.